Accurate level measurement is required for inventory control and custody transfer in industries where liquids such as oil, tar, chemicals and other materials are stored in large tanks. Large storage tanks may be on the order of 30 meters (100 feet) deep and 30 to 60 meters (100-200 feet) across. Process tanks may be on the order of 3 to 5 meters (10 to 17 feet) deep and 3 to 5 meters (10 to 17 feet) across. Historically, liquid levels have been measured by lowering a device into the tank from a roof-mounted platform. Devices used for this purpose have included long sticks and both manual and automated versions of weighted strings or wires which can be lowered into the tank until the liquid (or solid) content is touched. The depth of the material in the tank, and thus the volume of material in the tank, can then be readily determined from the length of the stick, string, or wire. The early use of sticks has led to the term "sticking the tank" for describing the level measurement procedure. While such mechanical methods have continuously been improved and can achieve a very high accuracy in some applications, all such methods require a physical intrusion of the tank environment. There is thus a potential for contamination of the tank contents and for the corrosion, and the ultimate destruction of the measuring device itself.
Over the past approximately fifteen years, various level measuring devices using radar techniques have been developed (see, e.g., U.S. Pat. No. 4,044,355 and U.S. Pat. No. 4,665,403). With the use of radar techniques the material to be measured is not touched; however, the antenna which radiates and receives microwave energy in order to make the measurement, is mounted inside the tank. In many tank environments, a corrosive gaseous atmosphere exists in the space above the liquid and the antenna material erodes. The antenna material thus contaminates the tank contents. These radar-type devices are also relatively expensive compared to the mechanical-type devices.
In some recently described devices, the antenna is separated from the tank contents by a transparent window. However, in these devices, the vertical profile is thus increased by the length of the antenna.
Typical radar-type level measuring devices employ horn antennas or parabolic reflector antennas which extend inside the tank, and use FM-CW (continuous wave) waveforms in which the frequency of the transmitted signal is linearly swept over about one (1) gigahertz (GHz). The received signal is mixed directly with the transmitted signal to form a mixer output signal whose frequency (the measuring frequency) is constant and is directly proportional to the distance from the antenna to the reflecting surface (the material whose level is to be measured). Such systems also employ various methods to attempt to correct for the non-linearity of the swept waveform, and hybrid analog/digital implementations of tracking filters which serve to process the signal such that the frequency of the mixer output signal can be correctly estimated and the liquid level can be determined.
Thus, what is needed is a level measuring method and apparatus which includes an antenna implementation which can be outside the tank interior atmosphere and have a low vertical profile. Such a system would desirably use a waveform which does not require sweeping the frequency of the transmitted waveform, and minimize part count by using a single receive path with one microwave mixer, one receiver amplifier, and one analog to digital (ND) converter. Such a system would also desirably use all digital processing after the first receiver amplifier to minimize contributors to the measurement error, and use digital processing algorithms to further minimize part count and improve reliability.